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TFC Selection for MAC Scheduling in WCDMA

Duan-Shin Lee and Chiung-Sui Liu

Department of Computer Science

National Tsing-Hua University, Hsinchu, Taiwan 30043

[email protected] and [email protected]

Abstract Reacting to the evolution of users needs toward mul-timedia applications, an important feature of the third generationmobile networks is to support efficiently multiple applications withdifferent quality of service. In the radio access networks of UMTS,RLC/MAC layers are designed to accommodate simultaneously mixedservices through establishing multiple bearers. A major issue is therate adaption of these bearers. In this paper, we examine the schedul-ing problem in the WCDMA MAC layer and propose five schedulingmethods. Our simulation result shows that a load measurement basedpriority method can achieve better fairness and has better executiontime performance than the other four methods.

Index Terms Transport Format Combination Selection, MACscheduling, WCDMA

I. INTRODUCTION

IN Radio Access Networks (RAN), Radio Link Control and

Medium Access Control layer have been designed to accom-

modate simultaneously mixed services including real-time and non

real-time traffic. This is achieved through establishing multiple

bearers at the same time. Therefore, a major issue is the rate adap-

tation of these bearers. Moreover, third generation radio interface

provides procedures to the rate adaptation in the lower layers. As

these procedures may occur in PHY, MAC or RLC layers, we pay

attention to MAC layer in the paper.

Further, the UMTS standard offers the UE the capability ofrunning multiple applications simultaneously through establishing

multiple logical channels. Each logical channel will be given a

priority value between 1(high) and 8(low). Logical channels are

responsible for transmitting the data traffic from various services

to MAC layer and will be multiplexed to transport channels. Then

the transport channels will manage to transmit the data traffic to

the physical layer. Moreover, the transport channels define the

ways how the data traffic from logical channels is processed and

sent to the physical layer. In other words, each transport channel

defines specific formats for transmitting the data traffic. And the

combinations of the formats of each transport channel are defined

by the network. However, we need to decide the format of eachtransport channel from the combinations provided by the network

to transmit data.

In this paper, our task is to schedule the provided resource to

the logical channels which are established for various applications

with different qualities of service. We propose five scheduling

methods which schedule logical channels according their priori-

ties and/or buffer occupancies. We consider the uplink data trans-

mission only. This paper is organized in the following way. In

section 2, we review the transport format combination selection in

WCDMA MAC layers. From section III to section VI, we present

the five scheduling methods. In sections VII and VIII we present

the simulation result and the conclusions of the paper.

This work is supported in part by Acer Mobile Network, Inc, (90A0255SB) andthe program for promoting academic excellence of universities (89-E-FA04-1-4).

II . TRANSPORT FORMAT COMBINATION SELECTION IN MAC

LAYERS

In UMTS radio networks, an UE has the ability to support mul-

tiple applications of different qualities of service running simulta-

neously in a WCDMA system. In the MAC layer, multiple logical

channels can be multiplexed to a single transport channel [2][5]. In

3GPP documents, the transport channel defines the way how traf-

fic from logical channels is processed and sent to physical layer.

The basic data unit exchanged between MAC and physical layer is

called Transport Block (TB)[4]. It is composed of an RLC PDU

and a MAC header. During a period of time called the transmis-

sion time interval (TTI), several transport blocks and some otherparameters are delivered to the physical layer. The set of specific

attributes forms a Transport Format (TF) of the considered trans-

port channel. They constitute of two parts, a dynamic part and a

semi-static part. The attributes of the semi-static part are the dura-

tion of time interval and coding parameters, such as the size error

correcting codes, coding types and coding rates. The dynamic part

of Transport Format forms the Transport Format Set (TFS) of the

considered transport channel. This allows a transport channel to

support different instantaneous bit rates. Each transport format in

the TFS will be identified as a Transport Format Indicator (TFI).

See Table I for an example. For each transport channel and for

each TTI, the MAC layer will choose an appropriate TF. As theremay be more than one transport channel, the combination of the

selected TFs for all transport channels forms the Transport Format

Combination (TFC) which will be identified as a Transport Format

Combination Indicator (TFCI). All the TFCs that an UE is permit-

ted to transmit during the transmission time interval are included in

a list called the Transport Format Combination Set (TFCS). TFCSs

are assigned by the network. See Table II for an example.

TFC selection is an important function in MAC layers in

WCDMA networks. A MAC layer will choose an appropriate TFC

in every TTI considering the status of the logical channels and the

provided radio resources of the transport channels. Each logical

channel is managed by a separate RLC entity, which is respon-

sible for executing segmentation and concatenation of data pack-ets to adapt to the size of a MAC PDU. Moreover, according to

3GPP documents, RLC layers can work in three modes: unac-

knowledged mode (UM), acknowledged mode (AM) and trans-

parent mode (TM) [3]. In this paper we only consider the unac-

knowledged mode only. Hence, the segmentation and concatena-

tion performed in the RLC layer will be based on the result of TFC

selection in the MAC layer. In addition, if the offered data from the

RLC layer cannot be segmented into multiple MAC PDUs exactly,

the last MAC PDU will be padded with redundant bits. However,

the WCDMA document does not allow a MAC PDU to contain

entirely padded redundant bits. We call this restriction the all-

redundant-bit-padding problem. This restriction will influence oursolution of the TFCS selection problem.

There are two problems that we need to solve in the transport


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format combination selection and scheduling problem. The first

problem is that we need to find a suitable transport format combi-

nation in the TFCS. This problem is nontrivial because the TFCS

contains only a subset of all the combination of the transport for-

mats of the transport channels and there is no specific rule on which

transport format combination is in the TFCS and which is not. Fur-

thermore, one needs to solve the transport format selection prob-

lem in one TTI time interval. This imposes a limit on the complex-

ity of the proposed scheduling methods. We call the first problem

the transport format combination selection problem. Recall thatthere can be multiple logical channels multiplexed into one trans-

port channel. After one selects a particular transport format com-

bination, one needs to assign the transport blocks of the transport

channels to their corresponding multiplexed logical channels. This

transport block assignment problem is the second problem that we

need to solve. We call the second problem the transport block

assignment problem. Recall that the WCDMA documents do not

allow all-redundant-bit-padding MAC PDUs. This is a restriction

in the transport block assignment problem.

III. STRICT PRIORITY METHOD

In this section, we schedule logical channels according to their

priority levels only. In this section, we show how this method canbe implemented in an efficient manner. Our objective is to select

a TFC from the provided TFCS and assign the transport blocks of

transport channels to the corresponding multiplexed logical chan-

nels in every TTI so that high priority logical channels have high

priority to transmit their data. Since the valid TFCS is a subset of

all the combination of the transport formats and the selected TFC

must be in the provided TFCS, we solve the strict priority schedul-

ing by disqualification and elimination. Assume that there are

transport channels. For transport channel

,

, let

be the set of logical channels served by

. Specifically, we iden-

tify the highest priority logical channel in the set

. Let this

channel be denoted by

and its associated transport channel bedenoted

. We examine

s TFs and identify the TF that allows

channel

to transmit as much information as possible. If there are

more than one such TF, choose one that corrsponds to a smaller

data rate. We do so because the TFs that have smaller data rates

require less power to transmit. Denote this TF by

. We solve

the transport block assignment problem for this TF according to

the priorities of the logical channels served by channel

. Then, in

TFCS we eliminate all TFCs except those that contain

. Then we

identify the highest priority logical channel in the set

.

We repeat the above procedure for this channel. After we repeat the

above procedure for all transport channels, we finish the schedul-

ing problem. Clearly, in the solution of this procedure the amount

of information that logical channels can transmit increases with the

priority. We refer the readers to [?] for more details.The 3GPP documents specify that all WCDMA equipment man-

ufacturers must implement this scheduling method in their radio

access network products. The main concern of this method is that

low priority logical channels can get starved if the high priority

channels have a lot of data to transmit. In the next four methods,

we take buffer occupancy levels into the scheduling consideration.

This should help to relieve the starvation problem of low priority

logical channels when the traffic load is high.

IV. DYNAMIC PRIORITY METHOD AND PARTIALLY DYNAMIC

PRIORITY METHODOne method to relieve the starvation problem of low priority

logical channels is to dynamically adjust the priority levels based

on buffer accumulation. Then, we apply the strict priority method

to schedule the logical channels according to their new adjusted

priority levels.

To this end, we set a buffer threshold to each logical channel.

For each logical channel, we compute the difference between its

queue length and its threshold. For any logical channel where the

queue length exceeds the threshold, the difference is positive. In

this case, the logical channel is considered to be congested and is

labelled with a mark H. Otherwise, it is in normal condition and

is labelled with a mark L. We arrange the marks of the logicalchannels in a list in the descending order of their original priorities.

See Fig. 1 for an example. We segment the list of logical channels

into one or more priority adjustment regions. We identify the posi-

tions where the marks of the logical channels change from H to

L as the left boundaries of priority adjustment regions. Similarly,

the positions where the marks of the logical channels change from

H to L as the right boundaries of priority adjustment regions.

Fig. 1. The priority adjustment regions of logical channels.

The partial dynamic priority method and the dynamic priority

method swap the positions of the L channels with those of the

H channels within priority adjustment regions. After the swap-

ping, the new positions identify the new priorities levels (in de-

scending order). The two methods differ in the way that channels

are swapped. In the dynamic priority method, the order of the log-

ical channels within a priority adjustment region is rearranged in

descending order according to their difference values between theirqueue lengths and thresholds. In the partially dynamic priority

method, we first determine the number of L channels and H

channels in a priority adjustment region. For a particular adjust-

ment region, let there be

H channels and

L channels.

Then, we swap the positions of

L channels

with those of equal number of H channels. Specifically, we se-

lect

out of

L channels that are the least congested com-

pared to their thresholds. Similarly, we select

out of

H

channels that are the most congested relatively to their thresholds.

The partially dynamic priority methods swap the positions of these

channels. For more details, we refer to [?].

V. PROBABILITY PRIORITY METHOD

In this section we present a scheduling method based on ran-

domization. The main objective of this method is to select a TFC in

an uncomplicated way and also make low priority logical channels

have some chances to transmit data. First, for each transport chan-

nel and its corresponding TFS, we examine all the TFs and delete

the TFs that violate the all-redundant-bits-padding constraint. We

also delete the transport format combinations that contain the in-

valid TFs in the TFCS. Then, we associate with each transport

channel a probability given by

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where

is the priority of channel

. For each transport channel,

say channel

, we draw a random number whose value is one with

probability

and is zero with probability

. The TFC

selection problem will be solved according the realization of this

sequence of random numbers for the transport channels. Specifi-

cally, let

be the set of transport format indicators of trans-

port channel

in the transport format combinations in the TFCS.

Clearly,

is a subset of

. Define the maximum rate TF

in

for transport channel

to be the TFI

(1)

where

and

are the number of transport blocks and the trans-

port block size of the

-th TF.

is the MAC header size of log-

ical channel

and

is the amount data for channel

to deliver

in the current TTI. Now we do the disqualification process for ev-

ery transport channel. If the realization of the random variable

corresponding to the

-th transport channel is 1, we delete all the

transport format combinations from the TFCS except for the TFCs

that contain only TFI

for transport channel

. If the realizationof the random variable is zero, we skip the disqualification of TFs

for this transport channel. After we finish the TF disqualification

for all transport channels, if there are multiple TFCs in the TFCS,

we choose the TFC that can transmit the most amount of infor-

mation. That is, we choose the TFC that has the maximum sum

of fraction of transport blocks number and the maximal required

transport block number over all transport channels. This solves the

TFC selection problem.

Now we describe how we solve the transport block assignment

problem by randomization. We repeat the following iterative pro-

cedure for every transport channel. To illustrate, assume that we

are assigning the blocks for transport channel

. Assume that the

TF of channel

in the selected TFC has

transport blocks with

block size

. Assume that the MAC header size is

. Initially

, and let set

be the set of logical channels that

are multiplexed to transport channel

. For iteration

, compute

probability

defined as

(2)

where

is the set of logical channels that are multiplexed to trans-

port channel . Now in the descending order of priority, draw a

random number for each logical channel in

sequentially. If the

random number is 1, try to assign

(3)

blocks to logical channel

, where

is the number of blocks that

have been assigned to logical channel

from iteration 0 up to iter-

ation

. In this case,

. If the sample value

of the random number is zero, assign 1 block to logical channel

. In this case,

. If

equals to

,

meaning that logical channel has acquired all the needed blocks,

then we let

. Finally, we increment the iterationindex by 1. We stop the iteration when we finish the assignment

of all the transport blocks.

V I. LOAD MEASUREMENT BASED PRIORITY METHOD

The weight of transport channel

is defined to be

where

is the set of logical channels that are multiplexed to trans-

port channel and

denotes the size of the set. Recall that

denotes the priority of channel . We let

denote the buffer

occupancy of channel at time . After we compute the weights

of all transport channels, we examine the transport channels in de-scending order according to their weights. For transport channel ,

we keep only the TFCs in TFCS which has the largest ratio in (1).

If there are multiple TFCs in the remaining TFCS, we choose the

TFC that has the lowest data rate. This solves the TFC selection

problem.We then estimate the packet arrival rates to the logical channels.

Assume that the system records the buffer occupancy, the selected

TFC and the number of transport blocks assigned to each logical

channel in the last

TTI time instances. Then we estimate the

packet arrival rate according to

where

denotes

. The predicted buffer occupancy in

the next time frame is

(4)

We will use the predicted buffer occupancies in (4) to assign the

transport blocks to the logical channels. The goal is to assign

more transport blocks to the logical channel that has large predicted

buffer occupancy and priority ratio. We do it iteratively. Assume

that the TF of transport channel

in the selected TFC has

trans-

port blocks with block size . Assume that the MAC header sizeis

. Let

, where

is the set of logical channels that

are served by transport channel

. In the

-th iteration, compute the

weight

where

is the predicted buffer occupancy of channel

in the

-th iteration and

. We also let

denote the

number of transport blocks to be assigned in iteration

. Clearly,

. Select the logical channel in

that have the largest

weight and assign to it

blocks. Then the number of transport blocks assigned to channel

up to iteration

is

where

is the numberof blocksthat have been assigned to chan-

nel

up to iteration

. Since channel

receives more transport

blocks, its predicted buffer occupancy becomes

The number of blocks to be assigned in iteration is

. If

equals

, logical

channel

has acquired all its needed blocks and we set

. Finally, we increment by one. We perform the above

procedure for all transport channels.

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VII. SIMULATION RESULTS

In this section, we will first introduce the simulation model.

Then we will present the simulation result. We use Poisson pro-

cesses as the traffic model. The priority level of logical channels

ranges from 1 (the highest) to 8 (the lowest). Each logical channel

may have different maximal buffer size. The threshold in the par-

tially dynamic priority method and the dynamic priority method is

selected to be fifty five percent of the maximum buffer size. The

load measurement based priority method measures the buffer oc-

cupancy based on the measurements in the last ten frames.In order to compare the performance of the proposed five

scheduling methods, we run transient and steady-state simulations.

In the transient simulation, we repeat 10000 independent simula-

tions with 1000 TTIs per simulation. In each steady-state simula-

tion, we simulate one million TTIs, where one TTI is 10 ms. In the

transient simulation, we show the variation of the buffer occupancy

of each logical channel of different priority through time. We also

show the variation of the weighted buffer occupancy of logical

channels. In the steady-state simulation, we compare the relation

of link utilization and weighted buffer occupancy and weighted

packet loss ratio caused by buffer overflow. The weighted buffer

occupancy is defined as

(5)

where

is the number of transport channels. The weighted loss

ratio is defined as

(6)

where

denotes the loss ratio of channel

. Finally, the link

utilization is defined as

(7)

according to the selection results in every TTI.

We examine the five scheduling methods by simulating two

TFCSs. The first TFCS is proposed by 3GPP in document [1].

We find that the performance of the five scheduling methods are

very similar for this TFS and TFCS. This is because the size of the

TFCS suggested by 3GPP for conformance testing is too small. We

omit the details due to space limit. We refer the readers to [?] for

details.

We construct a larger and more realistic TFCS to test the five

proposed scheduling methods. The TFS is shown in Table I andthe TFCS is shown in Table II. We consider two cases. In the

first case, there are three logical channels and in the second case,

there are five logical channels. In these two cases, the priority

levels of the logical channels equal to their indices. Specifically,

logical channel

has priority level

. In the first case, one logical

channel is served by exactly one transport channel. In the second

case, logical channel 1 and logical channel 5 are served by trans-

port channels 1 and 3, respectively. Logical channels 2, 3 and 4 are

multiplexed and served by transport channel 2. The packet arrival

rates and the packet lengths are shown in Table III. The weighted

buffer occupancy is shown in Fig. 2 and 3. From these figures,

we see that the load measurement based priority method has theleast weighted buffer occupancy. The weighted packet loss ratio of

the two cases is shown in Fig. 4 and Fig. 5. From this figure, we

Transport Channel

#1 #2 #3

BitRate(kbps) 79.8 96 91.5

TFS

TABLE I

THE TF S OF EACH TRANSPORT CHANNEL

(#1,#2,#3)=

(TF0,TF0,TF0),(TF0,TF1,TF0),(TF0,TF2,TF0),(TF0,TF0,TF1),

(TF0,TF0,TF2), (TF0,TF3,TF0),(TF0,TF0,TF3),(TF1,TF0,TF0),

(TF2,TF0,TF0),(TF3,TF0,TF0), (TF0,TF1,TF1),(TF0,TF1,TF2),

(TF0,TF2,TF1),(TF0,TF2,TF2),(TF1,TF1,TF0), (TF1,TF2,TF0),



(TF1,TF2,TF1), (TF1,TF2,TF2),(TF2,TF1,TF1),(TF2,TF1,TF2),

(TF2,TF2,TF1),(TF2,TF2,TF2), (TF0,TF1,TF3),(TF0,TF2,TF3),

(TF0,TF3,TF1),(TF0,TF3,TF2),(TF1,TF0,TF3), (TF2,TF0,TF3),


(TF3,TF0,TF1),(TF3,TF0,TF2)

TABLE II

THE TFCS

see that the load measurement based priority method has the least

weighted loss ratio. Since the load measurement based method es-

timates arrival rates by measurement, one may concern that as the

arrival rate changes abruptly the performance of the method may

degrade seriously. We conduct transient simulation on case 2. In

the transient simulation, we increase the arrival rate of channel 1

from 154 and 152 to 280 at time 6. The result is shown in Fig. 6.

This figure shows that the load measurement based method is quite

robust to abrupt load change.

Using simulation, we have estimate the execution time of the

five scheduling methods. The result is shown in Table IV. This

estimation is based on the second case and the TFCS in table I and

II. The simulation program was executed in the WINDOWS 2000

personal computer with a Pentium III 1GHz CPU. According to

Table IV, the load measurement based method has very reasonable

execution time compared to other methods.

case 1 Packet arrival rate Packet length Buffer size

(sec

) (bytes) (bytes)LCH 1 154 140 5000

LCH 2 170 250 5500

LCH 3 176 190 6000

case 2 Packet arrival rate Packet length Buffer size

(sec ) (bytes) (bytes)

LCH 1 152 140 5000

LCH 2 224 66 6000

LCH 3 190 78 7500

LCH 4 171 74 9000

LCH 5 175 190 10000

TABLE III

PACKET ARRIVAL RATES AND PACKET LENGTHS

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strict priority 0.1096 ms

partially dynamic 0.1413 ms

dynamic priority 0.1421 ms

probability priority 0.0216 ms

load measurement 0.0347 ms

TABLE IV

ESTIMATED EXECUTION TIME OF THE FIVE METHODS

0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

500

1000

1500

2000

2500

3000

3500

4000

Link Utilization

WeightedBOofLCHs

Compare WeightedBO with the 5 schemes

Strict priority methodLoad measurement based priority methodProbability priority methodPartially dynamic priority methodDynamic priority method

Fig. 2. The relation of weighted buffer occupancy and link utilization of Case 1.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1000

2000

3000

4000

5000

6000

7000

8000

Link Utilization

WeightedBO

ofLCHs

Compare weightedBO with the 5 schemes


Fig. 3. The relation of weighted buffer occupancy and link utilization of Case 2.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Link Utilization

WeightedLossRatio

Compare Weighted Loss Ratio with the 5 schemes

Strict priority method

Load measurement based priority methodProbability priority methodPartially dynamic priority methodDynamic priority method

Fig. 4. The relation of weighted loss ratio and link utilization of Case 1.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

Link Utilization

WeightedLossRatio

Compare Weighted Loss Ratio with the 5 schemes


Fig. 5. The relation of weighted loss ratio and link utilization of Case 2.

1 2 3 4 5 6 7 8 9400

500

600

700

800

900

1000

1100

1200

SimulationTime(sec)

WeightedBOofLCHs

Compare WeightedBO of LCHs using 5 schemes


Fig. 6. The transient weighted buffer occupancy of Case 2.

VIII. CONCLUSIONS

In this paper, we have studied five scheduling methods for TFC

selection to select an appropriate TFC and distribute transport

blocks to logical channels. From the simulation results, we find

that the load measurement based priority method has efficient per-

formance and maintains better fairness among logical channels

than the other four methods. The load measurement method has

excellent execution time performance as well.

REFERENCES

[1] Conformance testing v3.4.0. 3gpp, June 2001. TS 34.108.

[2] Mac protocol specification v.3.8.0. 3gpp, June 2001. TS 25.321.[3] Rlc protocol specification v.3.10.0. 3gpp, June 2001. TS 25.322.[4] Services provided by the physical layer v.3.9.0. 3gpp, June 2001. TS 25.302.[5] Harri Holma and Antti Toskala, editors. WCDMA in UMTS-Radio Access for

Third Generation Mobile Communications. Wiley, New York, 2000.

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